feat(calibration): 10-question battery, config-driven voice, WCAG-safe theme

Rework calibration.py into a fixed 10-question battery:
  - 4 critical questions (language, name, persona name, gender)
  - 6 random probes drawn from the 12-item pool (all AI/tech-unrelated)

Theme inference (palette × typography × density × labels):
  - Palette: warmth × contrast × energy → default|rose|morning|evening|
    sage|paper|ink; all choices verified WCAG AA ≥4.5:1.
  - Typography: elaborate+warm → serif-warm; cool+ink → mono;
    high-contrast+cool → serif-formal; energetic+warm → mixed-modern;
    otherwise sans.
  - Density: elaborate cadence → airy; terse → dense; else normal.
  - Labels: serif-warm/mixed-modern → cursive; mono → prefix; else block.

Voice selection:
  - Removed VOICE_POOL (lang × gender heuristic).
  - Added _load_persona_voices(): reads [voices.<lang>].id from a
    cart.toml persona file at PCART_CONFIG_PATH or app/conf/persona.toml.
    Falls back to FALLBACK_VOICES when no file is present.
  - _pick_voice(lang) selects by language key only — no gender heuristic.

cart_store.Cart changes:
  - Added calibration_version: int = 1 (new carts emit 2)
  - Bumped version: 3 → 4
  - Removed unused  import

No changes to app/main.py — calibration wiring was already correct.

app/cart_store.py

@@ -14,7 +14,7 @@ import json
 import logging
 import os
 import re
-from dataclasses import asdict, dataclass, field
+from dataclasses import asdict, dataclass
 from datetime import datetime
 from pathlib import Path
 
@@ -41,8 +41,10 @@ class Cart:
     ui_typography: str = "sans"       # sans | serif-warm | serif-formal | mixed-modern | mono
     ui_density: str = "normal"        # airy | normal | dense
     ui_labels: str = "block"          # block | cursive | none | prefix
+    # Tracks which calibration battery was used (bumped when calibration.py is reworked).
+    calibration_version: int = 1      # 1 = original critical+random; 2 = 10-question battery v2
     created_at: str = ""
-    version: int = 3
+    version: int = 4                  # bumped from 3 → 4 with calibration_version field addition
 
     @property
     def is_calibrated(self) -> bool: